Lithium ion secondary battery

ABSTRACT

A lithium ion secondary battery includes a positive electrode capable of absorbing and desorbing lithium ion, a negative electrode capable of absorbing and desorbing lithium ion, a porous film interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte: the porous film being adhered to a surface of at least one of the positive electrode and the negative electrode; the porous film including a filler and a resin binder; the resin binder content in the porous film being 1.5 to 8 parts by weight per 100 parts by weight of the filler; and the resin binder including an acrylonitrile unit, an acrylate unit, or a methacrylate unit.

CROSS REFERENCE TO RELATED APPLICATIONS

This application a Divisional of U.S. application Ser. No. 10/551,934,filed Oct. 4, 2005, now U.S. Pat. No. 7,396,612 which is the U.S.National Phase under 35 U.S.C. §371 of International Application No.PCT/JP04/010994, filed Jul. 26, 2004 which in turn claims the benefitsof Japanese Applications No. JP 2003-281419, filed Jul. 29, 2003, No. JP2003-341644, filed Sep. 30, 2003, No. JP 2003-343584, filed Oct. 1,2003, and No. JP 2004-025983, filed Feb. 2, 2004, the disclosures ofwhich Applications are incorporated by reference herein in theirentireties.

TECHNICAL FIELD

The present invention relates to a lithium ion secondary battery havinga porous film including a filler and a resin binder, and adhered to asurface of at least one of a positive electrode and a negativeelectrode. The present invention relates to an excellently safe lithiumion secondary battery in which a thermal runaway will not occur evenwhen an internal short circuit is caused.

BACKGROUND ART

As electronic devices become more and more portable and wireless, smalland lightweight lithium ion secondary batteries with high energydensity, are receiving attention as a power source for these devices.Lithium ion secondary batteries have a positive electrode comprising alithium-containing transition metal oxide or the like, a negativeelectrode comprising a carbon material or the like, and a non-aqueouselectrolyte.

In lithium ion secondary batteries, a separator is interposed betweenthe positive electrode and the negative electrode to electronicallyinsulate both electrodes from each other, and further to retain theelectrolyte. As the separator, a microporous membrane mainly composed ofa polyolefin such as polyethylene and polypropylene is used. Themicroporous membrane is formed by drawing a resin, generally.

However, such separator shrinks with heat under a comparatively lowtemperature of approximately 100° C. Therefore, a small short circuitmay rapidly expand to cause a thermal runaway. That is, when a shortcircuit is caused by an intrusion of a foreign substance or a nailpenetration test, the separator shrinks with the heat instantlygenerated. Based on such shrinkage, a damaged part of the separatorbecomes larger to expand the short circuit, causing the thermal runaway.Especially, when in an environment under a temperature over 150° C., thepossibility for impairing the battery safety based on the shrinkage of amicroporous membrane is high.

Thus, as schematically shown in FIG. 4, an attempt to make a pasteelectrolyte 40 to function as a separator. The paste electrolyte 40includes a great amount of liquid electrolyte 41 containing a thickener,and filler particles 42 having electrically insulating property: Thefiller particles 42 function as a spacer between a positive electrode 43and a negative electrode 44 (Japanese Laid-Open Patent Publication No.Hei 10-55718).

The paste electrolyte is a composite material of a liquid electrolytewith its viscosity increased by a thickener, and a filler havingelectrically insulating property. Therefore, the liquid electrolyte issufficiently included in the paste electrolyte, and the pasteelectrolyte is excellent in securing lithium ion conductivity of acertain level. However, the paste electrolyte has a defect ofimpracticality, since its strength as a separator is insufficient.

Also, it has been proposed to use a porous film comprising a filler anda resin binder and being adhered on a surface of at least one of apositive electrode and a negative electrode as a separator (JapaneseLaid-Open Patent Publication No. Hei 10-106530).

The porous film is formed by applying a raw material paste comprising afiller, and a resin binder dissolved in a solvent on a surface of anelectrode plate and then drying it. Such paste includes fluorocarbonresin, polyolefin rein, or the like as a resin binder.

Further, in order to prevent an inducement of a battery internal shortcircuit caused by a separation of a part of an electrode materialmixture from an electrode plate while manufacturing a battery, there hasbeen proposed to use a porous film and a separator such as the above incombination (Japanese Laid-Open Patent Publication No. Hei 7-220759).

The porous films described in Japanese Laid-Open Patent Publication No.Hei 10-106530 and Japanese Laid-Open Patent Publication No. Hei 7-220759are excellent to the extent that these can secure a certain level ofstrength and safety.

However, when a resin binder is dissolved in a solvent and thendeposited on a surface of filler particles, as schematically shown inFIG. 5, the area of filler particles 52 covered by a resin binder 51increases, which necessitates a usage of a great amount of resin binder.As a result, micropores among the filler particles decrease while thestrength increases, and paths for an electrolyte or lithium ion betweenthe positive electrode 53 and a negative electrode 54 tend to becomeinsufficient. That is, it is difficult to secure sufficient lithium ionconductivity, while maintaining a certain level of strength.

Additionally, since a resin having suitable properties for a resinbinder of porous film is not known, it is difficult to aim for a furtherimprovement of the strength of the porous film, while maintaininglithium ion conductivity.

DISCLOSURE OF INVENTION

The present invention relates to a lithium ion secondary battery havinga porous film including a filler and a resin binder and adhered to asurface of at least one of a positive electrode and a negativeelectrode.

One object of the present invention is to provide a lithium ionsecondary battery which can achieve both safety and high-ratecharacteristics, with a usage of a porous film which can secure heatresistance, necessary strength, and lithium ion conductivity by limitinga resin binder content in the porous film to a small amount, whileselecting a constituent monomer of the resin binder.

In order to improve lithium ion conductivity of porous film, microporeshave to be formed as much as possible in the porous film. Additionally,in order to form many micropores in the porous film, the amount of resinbinder relative to the amount of filler has to be as small as possible.However, even though many micropores are formed in the porous film, whenthe size of the micropores is inappropriate for the lithium iontransfer, lithium ion conductivity can not be improved to the maximum.In light of the above, one object of the present invention is to improvelithium ion conductivity of the porous film by controlling the averagepore size of micropores in the porous film.

Since tensile stress is applied on the electrode plate on which a porousfilm is formed at the time of forming the electrode plate group, theporous film might crack and cause a short-circuit failure. Althoughapplying conditions and drying conditions of a raw material pastecomprising a filler and a resin binder may give effects on thestress-tolerance of the porous film, in the end, the stress-tolerancefairly depends on the elongating percentage of the porous film. However,in order to secure lithium ion conductivity of the porous film, theresin binder content has to be limited to a small amount, so theattention is not given to the limitation of the elongating percentage.In light of the above, one object of the present invention is to improvereliability of a battery by controlling the elongating percentage of theporous film.

When the resin binder is limited to a small amount, although it isadvantageous in terms of discharge characteristics of a battery,strength of the porous film weakens and the porous film becomes apt tocrack. When the porous film separates from the electrode plate, itinduces an internal short circuit, and battery yields will decrease. Inparticular, in the case of wound type lithium ion secondary battery, apositive electrode and a negative electrode are wound around into aspiral shape interposing a separator between both electrodes. At theportion where the winding starts, radius of curvature is small toincrease bending stress, making the porous film apt to crack. In lightof the above, one object of the present invention is to suppress aninternal short circuit due to occurrence of material mixture separationin the manufacturing processes with a usage of the porous film whilemaintaining discharge characteristics of a battery, by controlling adistribution state of the resin binder in the thickness direction of theporous film.

The present invention relates to a lithium ion secondary battery:comprising,

a positive electrode capable of absorbing and desorbing lithium ion,

a negative electrode capable of absorbing and desorbing lithium ion,

a porous film interposed between the positive electrode and the negativeelectrode,

a non-aqueous electrolyte,

wherein the porous film is adhered to a surface of at least one of thepositive electrode and the negative electrode,

the porous film comprises a filler and a resin binder,

the content of the resin binder in the porous film is 1.5 to 8 parts byweight per 100 parts by weight of the filler, and

the resin binder includes an acrylonitrile unit, an acrylate unit, or amethacrylate unit.

The present invention also relates to a lithium ion secondary battery,wherein the average pore size of micropores in the porous film obtainedby a Bubble-point Method is 0.02 to 0.09 μm.

The present invention also relates to a lithium ion secondary battery,wherein the elongating percentage of the porous film is 15% or more.

The present invention also relates to a lithium ion secondary battery,wherein the amount of the resin binder is smaller in the first surfaceside where the porous film is in contact with the surface of theelectrode, and larger in the second surface side opposite to the firstsurface side.

It is preferable that the filler comprises a mixture of a large particlegroup and a small particle group, and that the average particle size Aof the large particle group and the average particle size B of the smallparticle group satisfy the formula (1):0.05≦B/A≦0.25.

It is preferable that the resin binder comprises rubber particles ofcore-shell type, and that the rubber particles have an adhesive surfaceportion.

It is preferable that the filler includes at least Al₂O₃.

It is preferable that the resin binder has the decomposition temperatureof 250° C. or more.

It is preferable that the resin binder has the crystalline melting pointof 250° C. or more.

The present invention also relates to a lithium ion secondary battery,wherein the porous film comprises a single film, and the amount of theresin binder gradually increases from the first surface side toward thesecond surface side.

The present invention also relates to a lithium ion secondary battery,wherein the porous film comprises a plurality of films, and the contentof the resin binder in the total of the filler and the resin binder in afilm positioned at the second surface side is higher than the content ofthe resin binder contained in the total of the filler and the resinbinder contained in a film positioned at the first surface side.

It is preferable that the filler content in the total of the filler andthe resin binder contained in a surface portion of the second surfaceside of the porous film is 70 to 98 wt %, when the thickness of thesurface portion is 20% of the thickness of the porous film.

The present invention relates to a lithium ion secondary battery inwhich the positive electrode and the negative electrode are wound in aspiral fashion interposing only the porous film.

The present invention also relates to a lithium ion secondary battery inwhich the positive electrode and the negative electrode are wound in aspiral fashion interposing the porous film and a separator.

The present invention relates to a method of manufacturing the abovelithium ion secondary battery, the method comprising the steps of:

(a) preparing a paste including 100 parts by weight of a filler, 1.5 to8 parts by weight of a resin binder including an acrylonitrile unit, anacrylate unit, or a methacrylate unit, and a dispersion medium of thefiller;

(b) applying the paste to a surface of at least one of a positiveelectrode and a negative electrode; and

(c) drying the paste applied on the surface of the electrode under atemperature of not less than 100° C. to not more than 180° C.

According to the present invention, a lithium ion secondary battery inwhich well-balanced heat-resistance, necessary strength, and lithium ionconductivity are secured, and in which both safety and high-ratecharacteristics are achieved can be provided, since the resin bindercontent in the porous film is limited to a small amount, and the resinbinder includes an acrylonitrile unit, an acrylate unit, or amethacrylate unit.

According to an embodiment of the present invention, a lithium ionsecondary battery excellent in discharge characteristics such ashigh-rate characteristics can be provided, since the average pore sizeof the micropores in the porous film is limited to 0.02 to 0.09 μm.

According to an embodiment of the present invention, a lithium ionsecondary battery which achieves both charge and dischargecharacteristics and reliability can be provided, since the elongatingpercentage is controlled, and the porous film having sufficienttolerance to stresses generated inside of the electrode plate group isused.

According to an embodiment of the present invention, sufficientmicropores for lithium ion to move to the electrode surface side can besecured while securing flexibility of the porous film, since the amountof the resin binder in the porous film is smaller in the first surfaceside in contact with the surface of the electrode, and larger in thesecond surface side opposite to the first surface side. Additionally,since the porous film has flexibility, separation of the porous film inmanufacturing processes is suppressed to prevent an internal shortcircuit. Therefore, a lithium ion secondary battery with higher qualityand safety can be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic illustration of a structure of a porous film inaccordance with the present invention.

FIG. 2 is a schematic illustration of an example of an arrangement of anelectrode to which a porous film in accordance with the presentinvention is adhered.

FIG. 3 is a schematic illustration showing a vertical cross section ofan example of the lithium ion secondary battery of the presentinvention.

FIG. 4 is a schematic illustration showing the structure of aconventional separator.

FIG. 5 is a schematic illustration showing the structure of anotherconventional separator.

FIG. 6 is an FT-IR absorption spectrum of an example of core-shell typerubber particles.

FIG. 7 is a schematic illustration showing a vertical cross section ofanother example of the lithium ion secondary battery of the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention relates to a lithium ion secondary battery:comprising, a positive electrode capable of absorbing and desorbinglithium ion, a negative electrode capable of absorbing and desorbinglithium ion, a porous film interposed between the positive electrode andthe negative electrode, and a non-aqueous electrolyte.

The porous film is adhered to a surface of at least one of the positiveelectrode and the negative electrode. And the porous film comprises afiller and a resin binder.

The amount of resin binder contained in the porous film is 1.5 to 8parts by weight per 100 parts by weight of the filler.

When the resin binder content is less than 1.5 parts by weight per 100parts by weight of the filler, a porous film having sufficient strengthcan not be obtained. Also, a porous film having a preferable elongatingpercentage can not be obtained.

On the other hand, when the resin binder content is over 8 parts byweight per 100 parts by weight of the filler, sufficient micropores cannot be formed in the porous film and rate characteristics will decline.Additionally, it becomes difficult to control the size of the microporesto the preferable range for the movement of lithium ion.

The resin binder includes an acrylonitrile unit, an acrylate unit, or amethacrylate unit.

In the case when an internal short circuit occurred, the temperature dueto the generated heat at the short circuiting portion will become about100° C. Therefore, when the decomposition temperature and crystallinemelting point of the resin binder are low, the porous film may deformand expand the short circuiting portion. In view of avoiding suchdefects, it is preferable that the resin binder has the decompositiontemperature of 250° C. or more. Also, when the resin binder iscrystalline, it is preferable that the resin binder has the crystallinemelting point of 250° C. or more. The crystalline melting point meansthe temperature when crystalline polymer starts softening.

Herein, it is preferable that the resin binder includes core-shell typerubber particles having an adhesive surface portion which can exertsufficient binding effects even with a small amount.

When using the core-shell type rubber particles, moving paths for theelectrolyte or lithium ion can be sufficiently secured, since fillerparticles can be adhered by points and more micropores can be secured inthe porous film. Also, the porous film can secure sufficient resistanceto stress.

The above state is schematically illustrated in FIG. 1. The fillerparticles 12 are adhered by points by means of core-shell type rubberparticles 11, and many micropores 15 are secured between a positiveelectrode 13 and a negative electrode 14. Therefore, since the movementof the electrolyte or lithium ion is not highly prevented, the lithiumion conductivity is secured sufficiently, and excellent ratecharacteristics can be maintained. Specifically, moving paths forlithium ion can be secured easily. Also, based on the point-adhesion,even with a usage of the rubber particles in small amount, strength andelongating percentage of a separator can be secured.

The average particle size of rubber particles is preferably 0.05 to 0.3μm, in terms of obtaining a porous film with well-balanced strength andporosity.

The adhesive surface portion of the core-shell type rubber particlespreferably includes an acrylate unit. As for the acrylate unit,2-ethylhexyl acrylate is preferable.

Conventionally, a resin material used for a battery is selected based onthe index for stability of a resin derived from the molecular orbitalmethod (HOMO/LUMO). Based on such index, generally, a resin with asingle composition or with a combination (copolymer) is selected.Therefore, a resin binder including an acrylonitrile unit which isunstable under a negative electrode potential is hardly selected fromthe conventional point of view.

When a plurality of kinds of resin binder are used in combination for aporous film, it is preferable that the ratio of core-shell type rubberparticles to the total amount of resin binders is 20 to 80 wt %.

When a plurality of resin binders are used in combination for a porousfilm as a resin binder, other than the core-shell type rubber particles,fluorocarbon resins such as polyvinylidene fluoride (PVDF), celluloseresins such as carboxymethyl cellulose (CMC), and polyvinyl pyrrolidone(PVP) can be used. Also, in view of giving appropriate viscosity to araw material paste of porous film, it is preferable that a fluorocarbonresin (for example, PVDF with a molecular weight of a hundred thousandto a million) is used in combination with the core-shell type rubberparticles.

In view of a balance between adhesion and rubber elasticity, it ispreferable that in the absorption spectrum of the core-shell type rubberparticles obtained by an FT-IR measurement, the absorption intensitybased on C═O stretching vibration is 3 to 50 times the absorptionintensity based on C≡N stretching vibration of the acrylonitrile unit.When the absorption intensity based on C═O stretching vibration is lessthan 3 times the absorption intensity based on C≡N stretching vibration,binding effects of the rubber particles become insufficient, and whenover 50 times, rubber elasticity of the rubber particles becomesinsufficient and the strength of the porous film is weakened. Theabsorption intensity refers to a height of the absorption peak from thebase line of the spectrum.

In the FT-IR measurement, absorption spectrum of the core-shell typerubber particles can be measured by using a sample in which the rubberparticles are applied on KBr plate, for example. Generally, theabsorption based on C═O stretching vibration is observed around 1700 to1760 cm⁻¹, and the absorption based on C≡N stretching vibration isobserved around 2200 to 2280 cm⁻¹.

It is preferable that the average particle size (median size D₅₀ basedon volume) of the filler is 0.2 to 2 μm. When the average particle sizeis too large, a thin (about a thickness of 20 μm, for example), uniformporous film can not be formed easily. When it is too small, necessaryamount of the resin binder will increase as the surface area of thefiller increases, and it becomes difficult to form sufficient microporesin the porous film.

Additionally, in view of bringing the packed state of the filler closeto a closest-packed state, or in view of making the adjustment of theaverage pore size of the micropores easier, it is preferable that thefiller includes a mixture of a large particle group and a small particlegroup. When stress is applied inside of the porous film inclosest-packed state, since the filler particles ease the stress basedon “sliding”, the film structure can be maintained easily even thoughthe elongating percentage became large.

It is preferable that the average particle size A (median size D₅₀ basedon volume) of the large particle group is 0.2 to 2 μm. Also, it ispreferable that the average particle size B (median size D₅₀ based onvolume) of the small particle group is 0.01 to 0.5 μm.

It is preferable that the average particle size A of the large particlegroup and the average particle size B of the small particle groupsatisfy the formula (1): 0.05≦B/A≦0.25. When the value B/A is below0.05, the surface area of the filler becomes too large, and it becomesdifficult to obtain a porous film having a sufficient strength by usinga small amount of resin binder. Or, since the surface area of the fillerbecomes large, the resin binder amount to be used becomes large, andmicropores in the porous film tend to decrease. On the other hand, whenthe value B/A becomes over 0.25, micropores formed among fillerparticles become too large, to suppress occurrences of capillarity todecline rate characteristics. Additionally, since the micropores to beformed among filler particles will become large, the sliding of thefiller will be suppressed to decrease the elongating percentage of theporous film.

It is preferable that the ratio of the small particle group to the totalamount of the filler is 1 to 20 wt %. And it is preferable that the restis the large particle group. When the ratio of the small particle groupis too small, it becomes difficult to charge the filler closely, andwhen the ratio of the small particle group is too large, the surfacearea of the filler becomes too large, to make it difficult to obtain aporous film having sufficient strength by using a small amount of resinbinder.

It is preferable that the filler comprises inorganic oxides including atleast an aluminum oxide (Al₂O₃). Other inorganic oxides such as titaniumoxide (TiO₂) and silicon oxide (SiO₂) may be used. These may be usedalone, or may be used in combination of two or more. However, it ispreferable that the ratio of Al₂O₃ to the total amount of the filler is50 wt % or more.

The reasons for using at least Al₂O₃ herein are: (1) the median size ofAl₂O₃ is suitable for the forming of a microporous structure (mediansize 0.02 to 0.09 μm) desired for the porous film; (2) it is stable withrespect to any of the oxidation and reduction potentials (0 to 5 V/vsLi); and (3) its particle surfaces are less uneven (surface area issmall), and the porous film having higher strength can be obtainedeasily by using a small amount of the resin binder.

By applying a raw material paste to a surface of at least one of thepositive electrode and the negative electrode, and then drying ifnecessary, the porous film can be obtained in the state being bonded tothe surface of the electrode. It is preferable that the drying iscarried out at 50 to 150° C. for 1 minute to 30 minutes. The dried filmformed on a surface of the electrode may be arbitrarily rolledafterwards to obtain a porous film.

The raw material paste for the porous film is prepared by dispersing thefiller and the resin binder in a liquid component. Water,N-methyl-2-pyrrolidone (hereinafter, NMP), acetone, a lower alcohol, andthe like may be used, and a non-aqueous electrolyte may be used for theliquid component at this time.

The raw material (the total of the filler and the resin binder) contentin the raw material paste of the porous film is preferably 25 to 70 wt%. When the raw material content is too small, it becomes difficult toform a porous film having a desired thickness and strength, and when theraw material content is too large, the application will be difficultbecause the paste viscosity will become high.

In a preferred embodiment of the present invention, the average poresize of the micropores of the porous film comprising the filler and theresin binder obtained by the Bubble-point Method is adjusted to 0.02 to0.09 μm.

Even though many micropores are secured in the porous film, when theaverage pore size of the micropores is below 0.02 μm, lithium ionconductivity will be insufficient, since transfer of solvated lithiumion is prevented by filler particles and resin binder. On the otherhand, when the average pore size of the micropores is over 0.09 μm,capillarity which accelerates transfer of lithium ion can not beutilized, thereby causing insufficient lithium ion conductivity as well.

Herein, the average pore size of the micropores in porous film may beobtained by a Bubble-point Method (ASTM F316-86, JIS K3832).Particularly, the average pore size d can be obtained as a median sizeby the following method.

(1) First, air pressure is applied from one side of the porous film indried state, and the relationship between the air pressure P (Psi) andthe flow rate of the air passing through the porous film (flow rate D indried state, unit: liter/min) is obtained.(2) Next, after a solvent (water, alcohol, or the like) is absorbed bythe micropores in the porous film, a contacting interface with thesolvent is formed on one side of the porous film and the air pressure isapplied from the rear side thereof, to obtain the relationship betweenthe air pressure P (Psi) and the flow rate of the air passing throughthe porous film (flow rate W in wet state, unit: liter/min). At thistime, the minimum pressure which allows occurrences of bubbles at thesurface of the porous film (bubble point) is set as P₀. At this time,the value W is 0, and when P reaches a certain value, the value of Dagrees with the value of W. The minimum pressure where the value of Dand the value of W coincide is set as P_(s).(3) In the pressure range from P₀ to P_(s) the relationship of P and Δ(W/D) is obtained, and converted to the relationship of d and Δ (W/D) byusing the relationship of d=0.451γ/P (γ: surface tension of water, unit:mN/m).(4) From the obtained converted value, the distribution of the flow ratepercentage Q defined as Q=Δ(W/D)×100 is obtained, and the value d isobtained as the median value of d in the distribution. Although theinterval of the value P at the time of obtaining the distribution is notparticularly limited, it is for example 150 to 250 Psi.

When obtaining the porous film having the micropores with the averagepore size of 0.02 to 0.09 μm calculated by the Bubble-point Method aswell, it is preferable that the resin binder includes the core-shelltype rubber particles having the adhesive surface portion.

The method for controlling the average pore size in the porous film isnot limited: The average pore size can be controlled by the viscosity ofthe raw material paste, the conditions for drying and rolling afterapplying the paste on a surface of an electrode, and the like.

Although the preferable conditions differ depending upon the kinds ofthe battery and it can not be generalized, in the case of the batterysame as the one in Example 1 mentioned later, for example, it ispreferable that the viscosity of the raw material paste is 1000 to100000 cP, the drying temperature after applying the paste on anelectrode surface is 45 to 200° C., and the line pressure of the rollingis about 1 to 1000 kgf/cm.

In a preferable embodiment of the present invention, the elongatingpercentage of the porous film comprising the filler and the resin binderis adjusted to 15% or more.

Although many micropores could be secured in the porous film, when theelongating percentage of the porous film is insufficient, the porousfilm can not endure the stress inside of the electrode plate group. Inview of suppressing an occurrence of a short circuit and obtaining ahighly reliable battery, it is required that the elongating percentageof the porous film is 15% or more.

When the elongating percentage is below 15%, the porous film may crackand the possibility for a short circuit to occur may increase when theelectrode plate are wound, for example. The cracking tends to occur atan innermost round of the winding where its curvature radius is thesmallest. The innermost round of the porous film has a diameter R ofabout 3 mm. Herein, the elongating percentage can be measured by themethod specified in JIS C 2318.

In a preferable embodiment of the present invention, the resin bindercontent in the porous film comprising a filler and a resin binder issmaller in a first surface side, and larger in a second surface side.The first surface refers to a surface of the porous film adhered to anelectrode surface, and the second surface refers to the opposite surfacethereof.

The porous film may comprise a single film, or may comprise a pluralityof films.

The porous film comprising a single film can be obtained by applying araw material paste of the porous film including a filler, a resinbinder, and a liquid component on an electrode, and then drying it. Byspeeding up volatilization of the liquid component with drying, theresin binder transfers to the second surface side of the porous filmwith the volatilization of the liquid component. As a result, from anelectrode surface toward the second surface side of the porous film, theresin binder content increases gradually. That is, in the thicknessdirection of the porous film, a concentration gradient is formed in theresin binder.

It is preferable that the drying temperature for the raw material pasteapplied on the electrode is not less than 100° C. to not more than 180°C. When the drying temperature is below 100° C., the speed forvolatilizing the liquid component lowers down to cause a uniformdistribution of resin binder concentration in the thickness direction ofthe porous film. On the other hand, the drying temperature of 180° C. ormore may cause an excessive amount of resin binder at the second surfaceside of the porous film. As a result, electrolyte absorption by theporous film or the electrode will be disturbed to decline dischargecharacteristics.

Next, the porous film comprising a plurality of films may be made by thefollowing method. First of all, a plurality of raw materials withdifferent resin binder contents for the porous film are prepared. A rawmaterial paste with a low resin binder content is applied on anelectrode and dried to form the first film. Afterwards, on the firstfilm, a raw material paste with a high resin binder content is appliedand dried to form the second film. When the porous film comprises threeor more films, the same operation is carried out by using a raw materialpaste with a further higher resin binder content. That is, raw materialpastes with different resin binder contents are applied, one by one froma material paste with low resin binder content to a material paste withhigh resin binder content, on an electrode and dried to form films.

The method of forming the porous film comprising a plurality of films isadvantageous in that it can voluntarily change the resin binder contentin each film, compared with the method of forming the porous filmcomprising a single film. The porous film also may be formed bylaminating a plurality of films respectively including different kindsof fillers.

In the method of forming the porous film including a single layer offilm, it is preferable that the resin binder is dissolved in the liquidcomponent. On the other hand, in the method of forming the porous filmcomprising a plurality of films, the resin binder is not necessarilydissolved in the liquid component. The resin binder just dispersed inthe liquid component, for example, may be appropriately used as well.

It is desirable that the filler content in the total of the filler andthe resin binder contained in the surface portion of the second surfaceside of the porous film is not less than 70 wt % to not more than 98 wt%, further desirably 90 to 98 wt %. However, the thickness of the“surface portion” herein is defined as 20% of the thickness of theporous film.

When the filler content is over 98 wt % and the resin binder content isbelow 2 wt % in the total of the filler and the resin binder containedin the surface portion of the second surface side of the porous film,cracks of the porous film may not be suppressed at the time of windingthe electrode plate.

Also, when the filler content is below 70 wt % and the resin bindercontent is over 30 wt % in the total of the filler and the resin bindercontained in the surface portion of the second surface side of theporous film, electrolyte absorption by the porous film or the electrodemay be disturbed.

The present invention may be applied to a lithium ion secondary batteryin which a positive electrode and a negative electrode are wound in aspiral fashion with only a porous film interposed therebetween, forexample. In this case, it is preferable that the thickness of the porousfilm is 10 to 50 μm, further preferably 10 to 30 μm, in view of fullydisplaying the function of the porous film to electronically insulatebetween the electrodes and to improve safety, while maintaining thedesigned capacity. It is also preferable that the thickness of theplurality of films in total is 10 to 50 μm, further preferably 10 to 30μm in the case of forming a porous film comprising a plurality of filmsas well.

The present invention can be also applied to a lithium ion secondarybattery in which a positive electrode and a negative electrode are woundin a spiral fashion with a porous film and a separator interposedtherebetween. In this case, although the thickness of the porous film isnot particularly limited, it is preferable that the thickness of theporous film is 0.5 to 20 μm, in view of fully displaying the function ofthe porous film to improve safety, while maintaining designed capacityof the battery. In the case of forming a porous film comprising aplurality of films as well, it is preferable that the thickness of theplurality of films in total is 0.5 to 20 μm. Further, it is preferablethat the total thickness of the separator and the porous film is 10 to50 μm, and further preferably 10 to 30 μm.

The present invention can be further applied to a battery in which apositive electrode and a negative electrode are not wound as in thebatteries above, but Just laminated. The thickness of the porous film isthe same as the thickness of the wound-type battery.

It is required that the separator comprises a material which is durableunder the environment in which the lithium ion secondary battery isused. For such materials, microporous membrane comprising a polyolefinresin such as polyethylene and polypropylene is generally used, althoughnot limited thereto. The microporous membrane may be a single layermembrane comprising one kind of polyolefin resin, or may be a plurallayer membrane comprising two kinds or more of polyolefin resin.

A positive electrode capable of absorbing and desorbing lithium ionusually comprises a positive electrode core material and a positiveelectrode material mixture carried thereon. The positive electrodematerial mixture generally includes a positive electrode activematerial, a binder, and a conductive agent.

For the positive electrode active material, a composite oxide is used.For the composite oxide, lithium cobaltate (LiCoO₂), modified lithiumcobaltates, lithium nickelate (LiNiO₂), modified lithium nickelates,lithium manganate (LiMn₂O₄), modified lithium manganates, and the likeare preferable. Each modified substance often includes elements such asaluminum and magnesium. Also, some composite oxides include at least twoof cobalt, nickel, and manganese.

For the binder included in the positive electrode material mixture,polytetrafluoroethylene, modified acrylonitrile rubber particles,polyvinylidene fluoride, and the like are used, for example, but notlimited thereto. It is preferable that the polytetrafluoroethylene andthe modified acrylonitrile rubber particles are used in combination withcarboxymethyl cellulose, polyethylene oxide, modified acrylonitrilerubber, and the like, which are to be a thickener for a raw materialpaste of a positive electrode material mixture. The polyvinylidenefluoride alone has both functions of a binder and a thickener.

The negative electrode capable of absorbing and desorbing lithium ionusually comprises a negative electrode core material and a negativeelectrode material mixture carried thereon. The negative electrodematerial mixture generally includes a negative electrode active materialand a binder, and includes a conductive agent and the like, whennecessary.

For the negative electrode active material, carbon materials such asvarious natural graphites, various artificial graphites, amorphouscarbon, composite materials including silicon such as silicide, variousalloy materials, and the like are used, for example.

For the binder to be included in the negative electrode materialmixture, polyvinylidene fluoride, modified polyvinylidene fluoride,styrene butadiene rubber, fluorocarbon resin, cellulose resin, and thelike are used.

For the conductive agent to be included in the positive electrodematerial mixture and the negative electrode material mixture, carbonblacks such as acetylene black and ketjen black, and various graphitescan be used.

The non-aqueous electrolyte generally comprises a non-aqueous solventand a lithium salt to be dissolved therein. It is preferable that thenon-aqueous electrolyte includes vinylene carbonate, cyclohexyl benzene,diphenyl ether, and the like as an additive.

As for the non-aqueous solvent, ethylene carbonate, dimethyl carbonate,diethyl carbonate, ethyl methyl carbonate, propylene carbonate,γ-butyrolactone, and derivatives thereof may be mentioned. These areused in combination of two or more in most cases.

For the lithium salt, lithium hexafluorophosphate (LiPF₆), lithiumtetrafluoroborate (LiBF₄), and the like are used, for example.

In the following, the present invention will be concretely explainedbased on Examples. However, the following Examples are not to limit thepresent invention.

Example 1

FIG. 2 and FIG. 3 are referred in the explanation.

(i) Fabrication of Positive Electrode

To 100 parts by weight of LiCoO₂, 4 parts by weight of polyvinylidenefluoride (PVDF) as a binder and 3 parts by weight of acetylene black asa conductive agent were added and subsequently an appropriate amount ofNMP (N-methyl-2-pyrrolidone) was added and then kneaded, to prepare apositive electrode material mixture paste.

The obtained positive electrode material mixture paste was applied onboth sides of an aluminum foil core material 21 with a thickness of 20μm, and then rolled so that the density of the active material (densityof LiCoO₂) in the positive electrode material mixture 22 became 3.3g/ml, to produce a positive electrode 23. A positive electrode lead 24made of aluminum was connected to the positive electrode 23.

(ii) Fabrication of Negative Electrode

To 100 parts by weight of spherical artificial graphite, 1 part byweight of styrene-methacrylic acid-butadiene copolymer as a binder and 1part by weight of carboxymethyl cellulose as a thickener were added andsubsequently an appropriate amount of water was added and then kneaded,to prepare a negative electrode material mixture paste.

Herein, for the styrene-methacrylic acid-butadiene copolymer as abinder, BM400B manufactured by ZEON Corporation was used.

The obtained negative electrode material mixture paste was applied ontoone side of a copper foil core material 25 with a thickness of 15 μm,and then rolled so that the density of the active material (density ofgraphite) in the negative electrode material mixture 26 became 1.4 g/ml,to produce a negative electrode 27. For the negative electrode 27, anegative electrode lead 28 made of copper was connected.

(iii) Formation of Porous Film

A raw material paste for porous film was prepared by mixing rawmaterials with the proportion shown in Tables 1 and 2. The raw material(the total of the filler and the resin binder) content in the paste wasset to become 50 wt % in any of the cases.

When BM500B was included in the resin binder, the filler and the resinbinder were dispersed or dissolved in NMP, and then kneaded to preparethe raw material paste.

When AD-211 was included in the resin binder, the filler and the resinbinder were dispersed or dissolved in water, and then kneaded to preparethe raw material paste.

Then, the raw material paste for porous film was applied with athickness of 20 μm on one side of the negative electrode 27 so that thenegative electrode material mixture 26 was completely covered with thepaste, to form a porous film 31. The exterior of the porous film wasthen observed, to check if peeling occurred or not.

TABLE 1 Amount of resin binder per 100 parts by weight Average particleHighest of filler size of filler High rate temperature (parts by weight)(μm) Peeling of characteristic reached Example BM500B PVDF Alumina aAlumina b porous film (%) (° C.) A1 2 2 0.4 — NONE 86.3 167 B1 0.75 0.750.4 — NONE 89.9 165 Com. 1a 0.5 0.5 0.4 — EXIST — 166 C1 3 3 0.4 — NONE84.1 167 D1 4 4 0.4 — NONE 80.5 169 Comp. 2a 5 5 0.4 — NONE 73.8 165 E12 2 0.4 0.05 NONE 87.8 166 (B/A = 0.125) F1 2 2 0.4 0.02 NONE 88.8 169(B/A = 0.05) G1 2 2 0.4 0.01 SLIGHTLY 89.2 168 (B/A = 0.025) H1 2 2 0.40.1 NONE 86.6 163 (B/A = 0.25) I1 2 2 0.4 0.15 NONE 85.1 166 (B/A =0.375) J1 2 2 0.2 — SLIGHTLY 85.8 164 K1 2 2 1 — NONE 86.4 166 L1 2 2 2— NONE 86.7 168 M1 0.8 3.2 0.4 — SLIGHTLY 85.6 172 N1 3.2 0.8 0.4 — NONE85.5 166 Com. 3a Microporous membrane — — — 88.7 188 O1 2 2 0.1 —SLIGHTLY 89.4 168 P1 2 2 5 — NONE 81.6 168

TABLE 2 Amount of resin binder per 100 parts by weight Average particlePeeling Highest of filler size of filler of High rate temperature (partsby weight) (μm) porous characteristic reached Example AD-211 CMC Aluminaa Alumina b film (%) (° C.) Q1 2 2 0.4 — NONE 86.4 166 R1 0.75 0.75 0.4— NONE 89.7 165 S1 4 4 0.4 — NONE 83.8 168

The raw materials are explained in the following.

[Resin Binder]

For the resin binder, core-shell type rubber particles, andpolyvinylidene fluoride (PVDF) with the molecular weight of 350,000 orcarboxymethyl cellulose (CMC) are used in combination.

Herein, for the core-shell type rubber particles, BM 500B manufacturedby ZEON Corporation or AD-211 manufactured by ZEON Corporation, eachincludes rubber particles comprising acrylonitrile-acrylate copolymer,were used. The average particle size of the rubber particles was 0.2 μm,respectively.

The absorption spectrum of the rubber particles (BM500B) obtained by anFT-IR measurement is shown in FIG. 6. FT-IR microscope (Continu μm,Light Source: AVATAR-360) manufactured by Nicolet Instrument Corporationwas used for the measurement device.

The measurement conditions were set as: sample scan 32 times, backgroundscan 32 times, resolution 4000, and sample gain 1.0. For the measurementsample, rubber particles were dispersed in NMP, applied on KBr plate,and dried to use.

In FIG. 6, the absorption peak observed near 2240 cm⁻¹ is based on C≡Nstretching vibration of acrylonitrile and absorption peak near 1733 cm⁻¹is based on C═O stretching vibration. In FIG. 6, absorption peakintensity (peak height) based on C═O stretching vibration is about 10times the absorption peak intensity (peak height) based on C≡Nstretching vibration of acrylonitrile unit.

The same FT-IR measurement result was obtained for the rubber particles(AD-211) as well.

[Filler]

For the filler, Al₂O₃ was used. Herein, “alumina a” with the averageparticle size of 0.4 μm was used alone, or a mixture of “alumina a” and“alumina b” with the average particle size 0.01 to 0.15 μm was used. The“alumina a” content and the “alumina b” content in the mixture were setas 90 wt % and 10 wt %, respectively. When the particle sizedistribution of the alumina mixture was measured, peaks of particle sizewere observed at 0.35 μm and at 0.2 μm or below, respectively.

(iv) Battery Assembly

Afterwards, as shown in FIG. 2, the positive electrode 23 was disposedon the porous film 31 to form a laminate-type unit cell comprising apair of positive electrode and negative electrode. This unit cell wasenveloped by an outer jacket 32 comprising aluminum laminate sheet, andthen, a non-aqueous electrolyte was charged into the outer jacket.

Herein, for the non-aqueous electrolyte, a solvent mixture in whichethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate weremixed with a volume ratio of 1:1:1, dissolving lithiumhexafluorophosphate (LiPF₆) to give a concentration of 1 mol/liter wasused. Also, 4 volume % of vinylene carbonate relative to the solventmixture was added to the non-aqueous electrolyte.

Then, resin sealing materials 33 covering a part of the positiveelectrode lead 24 and the negative electrode lead 28 are aligned to belocated at an opening end of the outer jacket 32, respectively, andouter jacket 32 was sealed in vacuum while free ends of the respectivelead were drawn to the outside. A lithium ion secondary battery with atheoretical capacity of 600 mAh as shown in FIG. 3 was thus completed.

For comparison, a battery was manufactured similarly for the case whereonly a conventional separator (thickness 20 μm) comprising microporousmembrane made of polyethylene instead of porous film was used.

(vi) Battery Safety

After each battery was activated by carrying out predetermined chargingand discharging, the batteries were charged with 120 mA until thebattery voltage reached 4.2 V, and then discharged with 60 mA until thebattery voltage reached 3 V. Then, the same charging was conducted, andthe temperatures of each battery with charged state were increased to160° C. The heating was stopped at 160° C., and the batteries were hungin midair. The heat generation behavior afterwards was measured with athermocouple. The highest temperatures reached of each battery at thistime are shown in Tables 1 and 2.

(vii) High Rate Characteristic of Battery

After each battery was activated by carrying out predetermined chargingand discharging, the batteries were charged with 120 mA until thebattery voltage reached 4.2 V, and then discharged with 60 mA until thebattery voltage reached 3 V. Then, the same charging was conducted, andthe batteries were discharged with 600 mA until the battery voltagereached 3 V. Afterwards, the ratio of the discharge capacity at the timeof discharging at 600 mA to discharging at 60 mA was obtained bypercentage. The results are shown in Tables 1 and 2.

(viii) Evaluation Results

From the results in Table 1, it is revealed that when the amount of theresin binder in the porous film is small, the porous film withsufficient strength can not be obtained due to occurrence of peeling ofporous film. Also, it is revealed that rate characteristics greatlydecline when the amount of the resin binder is excessive.

On the other hand, when the resin binder content in the porous film wasset as 1.5 to 8 parts by weight per 100 parts by weight of filler, ahigher safety and preferable high rate characteristic was obtained. Thisimplies that the mixing ratio of the filler and the resin binder isimportant. Also, this implies that the resin binder has preferableproperties for maintaining porous film strength while maintaininglithium ion conductivity.

Next, it is clear that the rate characteristic tends to graduallydecline as the ratio of the average particle size of “alumina b” to“alumina a” (value B/A) increases. On the other hand, it is clear thatwhen the value B/A is too small, the strength of the porous film tendsto decrease.

When the average particle size of the filler is too small, its surfacearea becomes large, and the porous film tends to peel off due toinsufficient resin binder. On the other hand, when the average particlesize of the filler is too large, the resin binder becomes excessive andtends to cause a decline in the high rate characteristic.

Example 2 (i) Fabrication of Positive Electrode and Negative Electrode

A positive electrode and a negative electrode were fabricated in thesame manner as Example 1.

(ii) Formation of Porous Film

A raw material paste of porous film was prepared in the same manner asExample 1. Herein, each raw material paste of porous film was preparedby dispersing a filler and a resin binder in NMP with the ratio shown inTable 3 and kneading them. The raw material content in the paste (atotal of the filler and the resin binder) was set as 50 wt % in any ofthe cases. For the filler, an alumina (Al₂O₃) with an average particlesize of 0.4 μm was used alone.

TABLE 3 Amount of resin binder per 100 Average Average pore parts byweight particle size of of filler size micropore in Peeling of High rate(parts by weight) of filler porous film porous characteristic ExampleBM500B PVDF (μm) (μm) film (%) A2 0.75 0.75 0.4 0.05 NONE 87.6 Com. 1b0.5 0.5 0.4 0.05 EXIST — B2 3 3 0.4 0.05 NONE 84.1 C2 4 4 0.4 0.05 NONE80.9 Com. 2b 5 5 0.4 0.05 NONE 76.5 Com. 3b 2 2 0.4 0.01 NONE 75.9 D2 22 0.4 0.02 NONE 84.0 E2 2 2 0.4 0.05 NONE 86.1 F2 2 2 0.4 0.07 NONE 85.2G2 2 2 0.4 0.09 NONE 82.9 Com. 4b 2 2 0.4 0.12 NONE 77.0 H2 0.8 3.2 0.40.05 NONE 85.8 I2 3.2 0.8 0.4 0.05 NONE 84.3 J2 2 2 0.2 0.05 SLIGHTLY87.2 K2 2 2 1 0.05 NONE 82.9 L2 2 2 2 0.05 NONE 80.8

As shown in FIGS. 2 and 3, the raw material paste of porous film wasapplied with a thickness of 20 μm on one side of the negative electrode27 so that the negative electrode material mixture 26 was completelycovered with the paste. A calendaring was conducted on the paste with apredetermined line pressure after drying, to form a porous film 31having micropores with the average pore size as shown in Table 3. Then,the exterior of the porous film was observed to check if peelingoccurred or not.

(iii) Measurement of Average Pore Size

The average pore size of the micropores was measured by a Bubble-pointMethod using Perm-porometer manufactured by Porous Material Inc. Waterwas used for the solvent at the time of measuring the flow rate W in wetstate.

Although the pore size distribution may be obtained by peeling off theporous film from the negative electrode fabricated for a measurement ofpore size, herein, the pore size distribution of the negative electrodewas obtained in advance, and then, the pore size distribution of thenegative electrode on which the porous film was formed was obtained.Then pore size distribution of only the porous film was obtained fromthe difference of these pore size distributions. A negative electrodeusually has micropores with a pore size of 0.5 to 5 μm, and a porousfilm usually has micropores with a pore size of 0.02 to 0.09 μm.Therefore, it is easy to calculate the pore size distribution of onlythe porous film.

Concretely, the flow rate D in dried state was obtained by applying anair pressure up to 250 Psi to the samples of the negative electrode orthe negative electrode on which a porous film was formed. Subsequently,the samples were wetted sufficiently by water, and then water was filledinto a container in which the samples were installed. The air pressureup to 250 Psi was applied to the samples, to obtain the flow rate W inwet state. The values D and W agreed in the range of 160 to 230 Psi inany of the samples.

In each sample, the distribution of flow rate percentage Q from thebubble point to the point where the value D equaled the value W wasobtained. The distribution in the porous film was obtained bysubtracting the distribution in the negative electrode from thedistribution in the negative electrode on which the porous film wasformed. The median value of the pore size d in the obtained distributionwas obtained as the average pore size of the micropores in the porousfilm.

The flow rate percentage Q is defined as the following. When the value Dequals the value W, the integrated value of Q becomes 100%.Q=Δ(W/D)=(Wh/Dh−Wl/Dl)×100

-   -   Wh: flow rate in wet state in high pressure side (Unit: L/min)    -   Wl: flow rate in wet state in low pressure side (Unit: L/min)    -   Dh: flow rate in dried state in high pressure side (Unit: L/min)    -   Dl: flow rate in dried state in low pressure side (Unit: L/min)

(iv) Battery Assembly

A lithium ion secondary battery as shown in FIG. 3 with a theoreticalcapacity of 600 mAh was completed in the same manner as Example 1 exceptthat the obtained porous film in which the average pore size of themicropores was adjusted was formed on the negative electrode.

(v) High Rate Characteristic of Battery

The high rate characteristic of each battery was evaluated in the samemanner as Example 1. The results are shown in Table 3.

(vi) Evaluation Results

From the results in Table 3 as well, it is clear that the porous filmhaving sufficient strength can not be obtained due to the peelingsoccurred in the porous film, when the amount of the resin binder in theporous film is small. When the amount of the resin binder is too large,it is clear that rate characteristic declines. That is, the results ofTable 3 imply that the resin binder content in the porous film should be1.5 to 8 parts by weight per 100 parts by weight of filler in order toobtain superior rate characteristic.

Next, it is clear that even the resin binder content in the porous filmand the average particle size of the filler are the same, the ratecharacteristic declines when the average pore size of the micropores inthe porous film is too small, or too large. That is, the results ofTable 3 imply that the average pore size of the micropores in the porousfilm should be 0.02 to 0.09 μm in order to obtain superior ratecharacteristic.

When the average particle size of the filler is too small, its surfacearea becomes large and the resin binder becomes insufficient, to causepeelings of porous film. On the other hand, when the average particlesize of the filler is too large, the high rate characteristic tends todecline. This is probably due to the fact that the resin binder becomesa surplus, and that the micropores having a pore size appropriate forthe transfer of lithium ion can not be obtained.

Example 3 (i) Fabrication of Positive Electrode

A positive electrode was fabricated in the same manner as Example 1except that the size of the electrode plate was changed to apredetermined size.

(ii) Fabrication of Negative Electrode

A negative electrode was made in the same manner as Example 1 exceptthat the negative electrode material mixture was carried on bothsurfaces of the copper foil core material to give a density of 1.4 g/mlof the active material (density of graphite), and that the size of theelectrode plate was changed to a predetermined size.

(iii) Formation of Porous Film

A filler and a resin binder were dispersed in NMP in proportions shownin Table 4, and then kneaded, to prepare a raw material paste of porousfilm. The raw material content in the paste (the total of the filler andthe resin binder) was set as 50 wt % in any of the cases.

As in Example 1, for the filler, the “alumina a” with an averageparticle size of 0.4 μm was used alone, or a mixture of the “alumina a”and the “alumina b” with an average particle size of 0.01 to 0.15 μm wasused. The “alumina a” content and the “alumina b” content in the mixturewere set as 90 wt % and 10 wt %, respectively.

Next, the raw material paste of porous film was applied on both sides ofthe negative electrode to cover the negative electrode material mixturecompletely with a thickness of 20 μm, and then dried for 20 minutes at90° C., to form a porous film. Then, the exterior of the porous film wasobserved to check whether a peeling occurred or not.

TABLE 4 Amount of resin binder per 100 parts by weight Average particleElongating of filler size of filler Peeling of percentage High rate(parts by weight) (μm) porous of porous Short Characteristic ExampleBM500B PVDF Alumina a Alumina b film film(%) circuit (%) A3 2 2 0.4 —NONE 18.6 NONE 85.7 B3 3.2 0.8 0.4 — NONE 15.1 NONE 88.0 C3 0.75 0.750.4 — NONE 15.6 NONE 89.2 D3 4 4 0.4 — NONE 20.8 NONE 83.1 E3 2 2 0.40.05 NONE 20.8 NONE 87.5 (B/A = 0.125) F3 2 2 0.4 0.02 NONE 20.4 NONE88.4 (B/A = 0.05) G3 2 2 0.4 0.1 NONE 19.5 NONE 86.1 (B/A = 0.25) H3 0 40.4 — NONE 10.3 EXIST — I3 0.5 0.5 0.4 — EXIST 7.7 EXIST — J3 5 5 0.4 —NONE 19.8 NONE 64.9 K3 2 2 0.4 0.01 EXIST 13.8 EXIST — (B/A = 0.025) L32 2 0.4 0.15 NONE 15.5 NONE 82.3 (B/A = 0.375)

(iv) Battery Assembly

FIG. 7 is referred to in the following description.

On a porous film 310 adhered to a surface of a negative electrode 270, apositive electrode 230 was disposed, and integrally wound to obtain acylindrical electrode plate group. Insulating rings 310 a and 310 b weredisposed on top and below of this electrode plate group, and then theelectrode plate group was inserted into a battery can 290 made of iron.Then, a positive electrode lead 240 and a negative electrode lead 280were welded to the inside of a sealing plate 300 and the inner bottomsurface of the battery can, respectively. Afterwards, a non-aqueouselectrolyte was charged into the battery can, and finally, the openingend of the battery can was caulked interposing a gasket 320 to the outeredge of a sealing plate 300. The same non-aqueous electrolyte as inExample 1 was used.

A lithium ion secondary battery as shown in FIG. 7 with a theoreticalcapacity of 2000 mAh (18650: diameter 18 mm, height 65 mm, cylindricalshape) was thus completed.

(v) Elongating Percentage of Porous Film

The elongating percentage of porous film was measured as in thefollowing, based on JIS C 2318.

First, the raw material paste of porous film used for each battery wasapplied on a film made of polyethylene terephthalate (PET) with athickness of 20 μm, and dried for 20 minutes at 90° C. Afterwards, theporous film after drying was peeled from the PET film, and the obtainedporous film was cut to give the size of 15 mm×25 mm, to serve as asample for measuring the elongating percentage.

The sample for measuring the elongating percentage was disposed in apredetermined tensile tester, and the test was conducted with a tensilespeed by which 5 mm of elongation was obtained per 1 minute. Then, theratio (%) of the elongation at the time when the sample broke relativeto the sample length (25 mm) was obtained. The results are shown inTable 4.

(vi) Presence or Absence of Short Circuit

For the batteries completed after the sealing, preliminary charge anddischarge were carried out by the patterns shown below, and thebatteries were stored for 7 days at 45° C.

Constant current charge: 400 mA (end voltage 4.0 V)

Constant current discharge: 400 mA (end voltage 3.0 V)

Constant current charge: 400 mA (end voltage 4.0 V)

Constant current discharge: 400 mA (end voltage 3.0 V)

Constant current charge: 400 mA (end voltage 4.0 V)

Before and after the above storing, voltages of each battery weremeasured, and it was determined that short circuits occurred in thosebatteries with drops in external voltage of not less than 70 mV afterthe storage. The results are shown in Table 4.

(vii) High Rate Characteristic of Battery

For those conforming batteries in which no short circuit occurred afterthe above storage for 7 days at 45° C., charge and discharge of thefollowing patterns were conducted under an atmosphere of 20° C.,afterwards.

<1> Preliminary Discharge

Constant current discharge: 400 mA (end voltage 3.0 V)

<2> First Pattern

Constant current charge: 1400 mA (end voltage 4.2 V)

Constant voltage charge: 4.2 V (end current 100 mA)

Constant current discharge: 400 mA (end voltage 3.0 V)

<3> Second Pattern

Constant current charge: 1400 mA (end voltage 4.2 V)

Constant voltage charge: 4.2 V (end current 100 mA)

Constant current discharge: 4000 mA (end voltage 3.0 V)

Then, the ratio of the discharge capacity at a discharge with 4000 mA tothe discharge capacity at a discharge with 400 mA was obtained bypercentage. The results are shown in Table 4.

(viii) Evaluation Results

From the results in Table 4, it is clear that when the amount of resinbinder in the porous film is small, the porous film peels, and theporous film with sufficient elongating percentage can not be obtained.Also, it is clear that the high rate characteristic greatly declineswhen the resin binder is excessive. That is, the results of Table 4 alsoimply that the resin binder content in the porous film should be 1.5 to8 parts by weight per 100 parts by weight of filler. Also, it is clearthat when the elongating percentage of porous film is below 15%, thepossibilities for a short circuit to occur will become high.

Next, it is clear that the elongating percentage of porous filmdecreases and high rate characteristic tends to decline gradually withthe increase in the ratio of average particle sizes of “alumina b” to“alumina a” (B/A value). On the other hand, it is clear that theelongating percentage of porous film tends to decrease when the B/Avalue is too small.

Example 4 Batteries 1 to 7

(i) Fabrication of Positive Electrode

A positive electrode material mixture paste was prepared by mixing 3 kgof lithium cobaltate, 1 kg of #1320 (NMP solution including 12 wt % ofpolyvinylidene fluoride) manufactured by Kureha Chemical Industry Co.,Ltd. as a binder, 90 g of acetylene black, and an appropriate amount ofNMP, with a double-arm kneader. This paste was applied on an aluminumfoil with a thickness of 15 μm, and rolled after drying, to form apositive electrode material mixture layer. At this time, the thicknessof an electrode plate comprising the aluminum foil and the materialmixture layer was set as 160 μm. Afterwards, the electrode plate was cutto give a width which could be inserted into a battery case with adiameter of 18 mm, and a height of 65 mm, to obtain a positive electrodehoop.

(ii) Fabrication of Negative Electrode

A negative electrode material mixture paste was prepared by mixing 3 kgof artificial graphite, 75 g of BM-400B (aqueous dispersion including 40wt % of stylene-butadiene copolymer) manufactured by ZEON Corporation asa binder, 30 g of carboxymethyl cellulose as a thickener, and anappropriate amount of water with a double-arm kneader. This paste wasapplied on a copper foil with a thickness of 10 μm, and rolled afterdrying, to form a negative electrode material mixture layer. At thistime, the electrode plate comprising the copper foil and the materialmixture layer was set as 180 μm. Afterwards, the electrode plate was cutto give a width which could be inserted into the above battery case, toobtain a negative electrode hoop.

(iii) Formation of Porous Film

In the batteries 1 to 7, a single porous film was formed on the negativeelectrode.

A raw material paste of porous film was prepared by putting 960 g ofalumina with a median diameter of 0.3 μm as an inorganic oxide filler,500 g of a modified acrylonitrile rubber (BM-720H manufactured by ZEONCorporation, solid content of 8 wt %, NMP 92 wt %) as a binder, and anappropriate amount of NMP into a double-arm kneader and mixing them.This paste was applied on both sides of the negative electrode, anddried with the drying conditions as shown in Table 5, to form a porousfilm with a thickness of 6 μm.

(iv) Battery Assembly

The positive electrode and the negative electrode having the porous filmwere wound interposing a separator with a thickness of 20 μm made ofpolyethylene microporous membrane, and then inserted into a batterycase. Then, 5.5 g of non-aqueous electrolyte was weighed and injectedinto the battery case, and the opening of the case was sealed. Acylindrical lithium ion secondary battery was thus made.

Herein, for the non-aqueous electrolyte, a mixed solvent of ethylenecarbonate, dimethyl carbonate, and methyl ethyl carbonate with a volumeratio of 2:3:3, dissolving lithium hexafluorophosphate (LiPF₆) to give aconcentration of 1 mol/L was used. Also, to the non-aqueous electrolyte,3 wt % of vinylene carbonate was added.

Battery 8

A battery 8 was fabricated in the same manner as the battery 1 exceptthat two porous films were formed on the negative electrode by thefollowing manner.

A raw material paste of porous film was prepared by putting 990 g of thesame alumina with the battery 1, 125 g of BM-720H as a binder, and anappropriate amount of NMP, into a double-arm kneader and mixing them.This paste was applied on both sides of the negative electrode, driedfor 10 seconds at 90° C., to form the first porous film with a thicknessof 4 μm.

Then, 980 g of the same alumina with the battery 1, 250 g of BM-720H asa binder, and an appropriate amount of NMP were put into a double-armkneader, and then kneaded, to prepare a raw material paste of porousfilm. This paste was applied onto the first porous film, and the pastewas dried for 10 seconds at 90° C., to form the second porous film witha thickness of 2 μm.

Battery 9

A battery 9 was fabricated in the same manner as the battery 1 exceptthat two porous films were formed on the negative electrode by thefollowing manner.

First, the first porous film was formed in the same manner as thebattery 8. Afterwards, a raw material paste of porous film comprising900 g of the same alumina as the battery 1, 1250 g of BM-720H as abinder, and an appropriate amount of NMP was prepared. This paste wasapplied onto the first porous film, and then dried for 10 seconds at 90°C. to form the second porous film with a thickness of 2 μm.

Battery 10

A battery 10 was fabricated in the same manner as the battery 1 exceptthat two porous films were formed on the negative electrode by thefollowing manner.

First, the first porous film was formed in the same manner as thebattery 8. Afterwards, a raw material paste of porous film comprising700 g of the same alumina with the battery 1, 3750 g of BM-720H as abinder, and an appropriate amount of NMP was prepared. This paste wasapplied on the first porous film, and dried for 10 seconds at 90° C. toobtain the second porous film with a thickness of 2 μm.

Battery 11

A battery 11 was fabricated in the same manner as the battery 1 exceptthat two porous films were formed on the negative electrode in thefollowing manner.

First, the first porous film same as the battery 8 was formed.Afterwards, a raw material paste of porous film comprising 600 g of thesame alumina with the battery 1, 5000 g of BM-720H as a binder, and anappropriate amount of NMP was prepared. This paste was applied onto thefirst porous film, and dried for 10 seconds at 90° C. to obtain thesecond porous film with a thickness of 2 μm.

Battery 12

A battery 12 was fabricated in the same manner as the battery 9 exceptthat titania was used instead of alumina as an inorganic oxide filler.

Battery 13

A battery 13 was fabricated in the same manner as the battery 9 exceptthat PVDF was used instead of BM-720H as a binder.

Comparative Battery 1

A comparative battery 1 was fabricated in the same manner as the battery1 except that two porous films same as the first porous film of thebattery 8 were laminated on the negative electrode.

Comparative Battery 2

A comparative battery 2 was fabricated in the same manner as the battery1 except that polyethylene (PE)-made beads were used instead of aluminaas a filler.

Comparative Battery 3

A comparative battery 3 was fabricated in the same manner as the battery1 except that no porous film was made on the negative electrode.

(v) Porous Film Strength

For the negative electrodes above, except for the negative electrode inthe comparative battery 3, the negative electrode having a porous filmwas wound around a round rod of φ5 mm as an axis. Afterwards, occurrenceof cracks on the porous film and the negative electrode were observed.The porous film and negative electrode which showed no chip, crack, andseparation were marked as “OK”, and the porous film and negativeelectrode which showed any of chip, crack, and separation were marked as“NG”. The results are shown in Table 5. Additionally, the conditions offormation of each battery are shown in Table 5. When the porous film andthe negative electrode showed “NG” in its strength, battery fabricationwas discontinued.

(vi) Discharge Characteristic

For the fabricated batteries, preliminary charge and discharge wereconducted by the patterns shown below, and the batteries were stored for7 days under an atmosphere of 45° C.

Constant current charge: 400 mA (end voltage 4.0 V)

Constant current discharge: 400 mA (end voltage 3.0 V)

Constant current charge: 400 mA (end voltage 4.0 V)

Constant current discharge: 400 mA (end voltage 3.0 V)

Constant current charge: 400 mA (end voltage 4.0 V)

Afterwards, the charge and discharge of the following patterns wereconducted under an atmosphere of 20° C.

<1> Preliminary Discharge

Constant current discharge: 400 mA (end voltage 3.0 V)

<2> First Pattern

Constant current charge: 1400 mA (end voltage 4.2 V)

Constant voltage charge: 4.2 V (end current 100 mA)

Constant current discharge: 400 mA (end voltage 3.0 V)

<3> Second Pattern

Constant current charge: 1400 mA (end voltage 4.2 V)

Constant voltage charge: 4.2 V (end current 100 mA)

Constant current discharge: 4000 mA (end voltage 3.0 V)

The results of the discharge capacity at this time are shown in Table 5.

(vi) Nail Penetration Test

For the batteries after the evaluation for discharge characteristic wasconducted, the following charges were conducted.

Constant current charge: 1400 mA (end voltage 4.25 V)

Constant voltage charge: 4.25 V (end current 100 mA)

An iron-made round nail of φ2.7 mm was penetrated into the batteriesafter charging under an atmosphere of 20° C. with the speed of 5mm/second from the side face of the battery, and heat generation at thetime was observed. The temperatures reached after 1 second and after 90seconds at the penetration point of the battery are shown in Table 5.

TABLE 5 Porous Film Nail Penetration Filler Discharge Test First SecondCharacteristic Highest Layer Layer Drying Drying Porous DischargeTemperature Layer Content Content Binder Temp. Time Film Capacity (mAh)After 1 After 90 Example Structure Kind (%) (%) Kind (° C.) (sec)Strength 400 mA 4000 mA Second Seconds 1 One Alumina 96 BM-720H 100 10OK 2010 1869 63 88 2 Layer 120 OK 2012 1891 64 87 3 140 OK 2013 1882 6486 4 160 OK 2013 1852 63 89 5 180 OK 2012 1831 62 88 6 200 OK 2009 181177 104 7 90 OK 2010 1790 75 102 8 Two 99 98 OK 2011 1890 65 89 9 Layers99 90 OK 2010 1829 64 87 10 99 70 OK 2012 1790 63 88 11 99 60 OK 20091740 69 88 12 Titania 99 90 OK 2013 1832 71 90 13 Alumina 99 90 PVDF OK2009 1859 72 146 Comp. 99 99 BM-720H NG Not Not Not Not battery 1dEvaluated Evaluated Evaluated Evaluated Comp. PE 99 90 OK 2012 1851 146Not battery 2d beads Evaluated Comp. Not Made — — Not 2012 1710 146 Notbattery 3d Evaluated Evaluated(vii) Evaluation Results

When the drying temperature is 90 to 200° C., and only one layer ofporous film with 96 wt % of alumina was formed on the negativeelectrode, as in the batteries 1 to 7, the results for porous filmstrength, discharge characteristic, and nail penetration test were allexcellent compared with the comparative battery 1. Also, the dischargecharacteristic of the batteries 1 to 7 was fine compared with thecomparative battery 3.

As for the battery 6 with the drying temperature of 200° C., regardingthe discharge characteristic under the constant current 4000 mA,discharge capacity was low, and in the nail penetration test, thehighest temperature reached after 90 seconds was high, compared with thebatteries 1 to 5. The battery 7 with the drying temperature of 90° C.had a higher “highest temperature reached after 90 seconds” in the nailpenetration test, compared with the batteries 1 to 5.

Thus, for the batteries 1 to 7, the alumina content in the thicknessdirection of the porous film was analyzed. As a result, it was revealedthat the alumina content in the surface side of the porous film becamesmall and the binder increased when the drying temperature was higher.Especially, in the case of the battery 6 with the drying temperature of200° C., the alumina content in the surface side of the porous film was60 wt %. It is considered that the discharge characteristic declines byan increase in the amount of the binder at the surface side of theporous film and an electrolyte absorptance is disturbed, when the dryingtemperature is 200° C.

Also, in the case of battery 7 with the drying temperature of 90° C.,the alumina content in the surface side of the porous film was 95.5 wt%, and there was a slight difference from the alumina content in theentire porous film of 96 wt %. Therefore, difference between the fillercontent of the surface side of the porous film and the filler content inthe entire porous film is desirably 1 wt % or more. Also, in view of thedischarge characteristic and the nail penetration test, it is preferablethat the drying temperature for the case where a single layer of porousfilm is formed on the electrode is set to 100 to 180° C.

As in the batteries 8 to 11, when the alumina content of the secondlayer of the porous film was changed in the range of 60 to 98 wt %, goodresults were obtained with regard to porous film strength, dischargecharacteristic, or nail penetration test, compared with the comparativebatteries 1 and 3. The battery 11 had a low discharge capacity withregard to discharge characteristic at a constant current of 4000 mA,compared with the batteries 8 to 10. This is probably due to the factthat the micropores in the filler could not be obtained sufficiently,and supply of the electrolyte to the electrode became insufficientbecause of an excessive amount of binder in the second layer of porousfilm.

Also, as in the comparative battery 1, when two porous films with 99 wt% alumina were formed on the negative electrode, separation of negativeelectrode material mixture was observed in the strength test of theporous film by winding and therefore the battery was not fabricated.Thus, in view of porous film strength and discharge characteristic, itis preferable that the inorganic oxide filler content in the surfaceside of the porous film is in the range of 70 to 98 wt %.

When PE-made beads were used as in the comparative battery 2, the nailpenetration test showed the same results with the case where porous filmwas not provided. This implies that the intended effects of the presentinvention can not be exerted when a filler having the same degree ofheat-resistance with the microporous membrane serving as a separator wasused. Therefore, an inorganic oxide has to be selected for the filler.

When titania was used instead of alumina, as in the battery 12, the sameeffects as in the case of alumina were confirmed. Based on this, it isclear that an inorganic oxide filler other than alumina can be used.

When the porous film comprising PVDF was used for the binder, as in thebattery 13, although the temperatures reached after 1 second in the nailpenetration test were almost the same as other Examples, the temperaturereached after 90 seconds were higher. As a result of disassembling thisbattery, although the existence of the porous film was confirmed, theshort circuiting part was large compared with the batteries 1 to 12.Based on this, as a binder for porous film, it is preferable that abinder which is not easily burned down or melted, particularly a binderhaving a decomposition temperature of 250° C. or more and a crystallinemelting point of 250° C. or more, is used. For example, it is preferablethat an amorphous polymer with rubber-like characteristics including anacrylonitrile unit with a decomposition temperature of 320° C. is usedas a binder.

When there was no porous film as in the comparative battery 3, thetemperatures reached after 1 second showed higher values compared withthe case where the porous film was formed on the electrode as in thebatteries 1 to 13 and the comparative battery 1. As a result ofdisassembling these batteries after the test, in the comparative battery3, the separator melted in a wide range. On the other hand, in thebatteries 1 to 13 and the comparative battery 1, the porous film existedon the electrode in the state as it was just manufactured, andheat-shrinkage of the separator was suppressed as well. Base on this, itcan be considered that the porous film was not damaged, expansion of theheat generating part due to a short circuit was suppressed, and athermal runaway was prevented, by using the porous film comprising abinder with a high melting temperature, even when heat was generated bya short circuit at the time of nail penetration.

Herein, characteristics of the nail penetration test, which is asubstitutional evaluation for an internal short circuit, and its dataanalysis are explained. First, causes for the heat generation by a nailpenetration can be explained as in the following, based on the resultsfrom the past tests.

When a positive electrode and a negative electrode make a partialcontact (a short circuit) by a nail penetration, a short circuit currentflows in to generate the Joule heat. Then, a separator material with alow heat resistance is melted by the Joule heat to expand the shortcircuiting part. As a result, the Joule heat is continued to begenerated, and the damages in the separator expand by a heat-shrinkage.Based on this, the temperature of the positive electrode is increased tothe temperature range (160° C. or more) where the positive electrodebecomes thermally unstable. The thermal runaway is thus caused.

Although the case where a porous film is formed on the negativeelectrode was explained in Examples, the same effects can be obtainedeven the porous film was formed on a positive electrode, or on bothelectrodes. Also, in Examples, although the case where a single or twoporous films were formed on the negative electrode was explained, thelayer can be 3 or more, which can obtain the same effects with theExamples.

INDUSTRIAL APPLICABILITY

The present invention is extremely useful in a field of lithium ionsecondary battery, for which safety, excellent charge and dischargecharacteristics, and high rate characteristic are required. The lithiumion secondary battery of the present invention is useful as a powersource for electronic devices such as laptop computers, cellular phones,and digital still cameras.

1. A lithium ion secondary battery comprising: a positive electrodecapable of absorbing and desorbing lithium ion; a negative electrodecapable of absorbing and desorbing lithium ion; a porous film interposedbetween said positive electrode and said negative electrode; and anon-aqueous electrolyte; wherein said porous film is adhered to asurface of at least one of said positive electrode and said negativeelectrode, said porous film comprises a filler and a resin binder, acontent of said resin binder in said porous film is 1.5 to 8 parts byweight per 100 parts by weight of said filler, and an amount of saidresin binder is smaller in a first surface side where said porous filmis in contact with said surface of said electrode, and larger in asecond surface side opposite to said first surface side, wherein acontent of said filler in the total of said filler and said resin bindercontained in a surface portion of said second surface side of saidporous film is 70 to 98 wt %, and a thickness of said surface portion is20% of the thickness of said porous film.
 2. The lithium ion secondarybattery in accordance with claim 1, wherein said resin binder comprisesrubber particles of core-shell type, and said rubber particles have anadhesive surface portion including at least an acrylonitrile unit, anacrylate unit, or a methacrylate unit.
 3. The lithium ion secondarybattery in accordance with claim 1, wherein said filler includes atleast Al₂O₃.
 4. The lithium ion secondary battery in accordance withclaim 1, wherein said resin binder has a decomposing temperature of 250°C. or more.
 5. The lithium ion secondary battery in accordance withclaim 1, wherein said resin binder has a crystalline melting point of250° C. or more.
 6. The lithium ion secondary battery in accordance withclaim 1, wherein said porous film comprises a single film, and an amountof said resin binder gradually increases from said first surface sidetoward said second surface side.
 7. The lithium ion secondary battery inaccordance with claim 1, wherein said filler comprises a mixture of alarge particle group and a small particle group, and an average particlesize A of said large particle group and an average particle size B ofsaid small particle group satisfy the formula (1):0.05≦B/A≦0.25.
 8. A lithium ion secondary battery comprising: a positiveelectrode capable of absorbing and desorbing lithium ion; a negativeelectrode capable of absorbing and desorbing lithium ion; a porous filminterposed between said positive electrode and said negative electrode;and a non-aqueous electrolyte; wherein said porous film is adhered to asurface of at least one of said positive electrode and said negativeelectrode, said porous film comprises a filler and a resin binder, acontent of said resin binder in said porous film is 1.5 to 8 parts byweight per 100 parts by weight of said filler, and an amount of saidresin binder is smaller in a first surface side where said porous filmis in contact with said surface of said electrode, and larger in asecond surface side opposite to said first surface side, wherein saidporous film comprises a plurality of films and a content of said resinbinder in the total of said filler and said resin binder contained in afilm positioned at said second surface side is higher than a content ofsaid resin binder in the total of said filler and said resin bindercontained in a film positioned at said first surface side.
 9. Thelithium ion secondary battery in accordance with claim 8, wherein saidresin binder comprises rubber particles of core-shell type, and saidrubber particles have an adhesive surface portion including at least anacrylonitrile unit, an acrylate unit, or a methacrylate unit.
 10. Thelithium ion secondary battery in accordance with claim 8, wherein saidfiller includes at Al₂O₃.
 11. The lithium ion secondary battery inaccordance with claim 8, wherein said resin binder has a decomposingtemperature of 250° C. or more.
 12. The lithium ion secondary battery inaccordance with claim 11, wherein said resin binder has a crystallinemelting point of 250° C. or more.
 13. The lithium ion secondary batteryin accordance with claim 8, wherein said filler comprises a mixture of alarge particle group and a small particle group, and an average particlesize A of said large particle group and an average particle size B ofsaid small particle group satisfy the formula (1):0.05≦B/A≦0.25.